fix(Employee): add/delete user permission (backport #47016)

[mergify]

• Checking write permissions on User Permission is incorrect, because we will either create or delete them.
• The called functions add_user_permission / remove_user_permission will do the correct permission checks.
• If a user tries to change the value of User ID or Create User Permission but does not have permissions to add/remove User Permission, they should get an error. Otherwise we'll get inconsistent system states (e.g. an Employee has Create User Permission enabled, but the corresponding User Permissions are missing).

Alternative: ignore permissions altogether, HR users need to change these fields but shouldn't have permissions on User Permission otherwise.

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This is an automatic backport of pull request #47016 done by Mergify.

[mergify]

Cherry-pick of 046bcfa has failed:

On branch mergify/bp/version-15-hotfix/pr-47016
Your branch is up to date with 'origin/version-15-hotfix'.

You are currently cherry-picking commit 046bcfa606.
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Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/setup/doctype/employee/employee.py

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Cherry-pick of 08f21c7 has failed:

On branch mergify/bp/version-15-hotfix/pr-47016
Your branch is ahead of 'origin/version-15-hotfix' by 1 commit.
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You are currently cherry-picking commit 08f21c7905.
  (fix conflicts and run "git cherry-pick --continue")
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Changes to be committed:
	modified:   erpnext/hooks.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/setup/doctype/employee/employee.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally